Thursday, February 4, 2010

Under Pressure

Prior to my present format (question/answer) in enRoute magazine, I would write facts and trivia on a particular aviation topic. I would rattle off about 25 to 30 facts and the editor would make their choice. Here's lots on aircraft pressurization:

Pressure Points:

Percentage of air pressure at 35,000 feet compared to sea level pressure:

Height of ‘cabin altitude’ when at cruising altitude:
6000 to 8000 feet

Equivalent altitude ambiance:
dining in the “Alps”

Height where half of earth’s atmosphere exists:
18, 000 feet

Number of altimeters in flight deck:
three: Captain’s, first officer’s, and stand-by

Rate cabin depressurizes when aircraft descends at 1500 feet/minute:
about 300 feet/minute

When pressurized flight began:
mid 1930’s

First pressurized aircraft:
Boeing Stratoliner

Where air comes from to pressurize aircraft:
air “bled” from engines

Equipment used to condition hot engine air to room temperature:

Number of packs per aircraft:

How air is extracted to outside:
through outflow valves

Typical relative humidity of cabin air:
about 10 percent

Relative humidity at Palm Springs, California:
about 10 percent

Innovation Air Canada’s new Boeing 787 Dreamliner will have:
add moisture to cabin air

Exchange rate of Boeing 767 cabin air:
once very six minutes

Number of passengers who experience some discomfort:
about 1 out of 3

Why some experience slight pain in descent:
ear valves have harder time to depressurize

Some contributing factors:
sinus problems or head colds

Why babies are more susceptible:
Eustachian tubes in inner ear are very narrow

What some passengers do by closing mouth, pinching nose and breath through nostrils:
Valsalva manoeuvre

Another remedy:
chew gum or candy

How pressure is regulated through flight:
automatic pressure controllers

How sensitive are these?
Pilots can see small rate change on gauge when toilet flushed on Airbus

Three pressure units pilots use:
inches of mercury, millibars (hectopascals), p.s.i (pounds per square inch)

What one square inch column of air would weigh at sea level:
14.7 pounds

Standard atmospheric pressure:
29.92 inches of mercury or 1013 millibars

Pressure setting pilots set on altimeter when at cruising height: 29.92

When this is set in North America:
above 18,000 feet

Equivalent unit for millibar:

Why pilot weather charts differ: weather depicted at different pressure levels

I received a recent email about cabin doors during pressurized flight. Can they be opened? The answer, NO! Why? Because of the gargantuan forces. It would be similar to the force required to jump up at the precise moment when a runaway eleveator hits the bottom of the elevator shaft or the force required to jump away from a nosediving airplane with a parachute. Lets do a back-of-an-envelope calculation:

But to answer this question, you would have to have Herculean strength. We all know airliners are pressurized. Many people assume they are pressurized to sea level. Not the case. You are basically sitting in cabin pressure equivalent to pressure found on a mountain 6,000 to 8000 feet high. Here's some math: The pressure differential (outside to inside) is about 7 P.S.I (pounds per square inch). Lets assume the door is about three feet wide by 6.5 feet tall. Around 20 square feet. Each square foot is 144 square inches. So a door would have an area of 144 x 20 = 2880 square inches. 2880 square inches x 7 pounds/square inch equals an amazing 20,160 pounds of force. Most cabin doors are a "plug" type so they push against the fuselage during closure and pressurization.

When I visit the galley and see the flight attendant sitting in their jumpseat next to the door, it makes me ponder a little, but when you think physics it sets the mind at ease.


Joe d'Eon said...

I knew a flight attendant who, while sitting next to the 1L door, got hit by severe turbulence. She grabbed the door handle to keep from hitting the ceiling, and the handle ... ripped off its mounts! (Also her shoulder was torn up pretty bad inside)

From the Flight Deck said...

Joe. That's quite an event the flight attendant went through. They tend to get hurt the most when turbulence arises.

Your story confirms the door and handle ain't going to move in a pressurized cabin.

Thanks for your post.

Captain Doug